Try the Firefox Safe Mode to see how it works there. The Safe Mode is a troubleshooting mode, which disables most add-ons.

(If you're not using it, switch to the Default theme.)

You can open the Firefox 4.0+ Safe Mode by holding the Shift key when you use the Firefox desktop or Start menu shortcut.

Or use the Help menu item and click on the Restart with Add-ons Disabled... menu item while Firefox is running.

Don't select anything right now, just use "'Start in Safe Mode"To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut (without the Shift key) to open it again.If it is good in the Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one.

You can either delete those two folders or only the key it.
Windows will create a new MRU key when those are missing and removing the keys is easier then clearing up a long list.
A lot of keys can get cluttered there over time.

Did you already try to remove the mimeTypes.rdf file?

The mimeTypes.rdf file stores file actions that you see in Tools > Options > Applications and it is possible that there is some interference.

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